hydraulics 11

HYDRAULICS

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Hydraulics

• Teaching Objectives– Learn basic concepts

• Water properties• Open channel flow• Culvert (or Pipe) flow• Variation in hydraulic conditions• Temperature and ice effects• Flow measurements

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Hydraulics Definition• Wikipedia: Hydraulics is a topic in applied science and

engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties.

• From Elementary Fluid Mechanics, Vennard and Street, 1975. “Because of the conflict between theory and experiment, two

schools of thought arose in the treatment of fluid mechanics, one dealing with the theoretical and the other with the practical aspects of fluid flow. In a sense, these two schools of thought have persisted to the present day, resulting in the mathematical field of hydrodynamics and the practical science of hydraulics.”

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DESCRIPTION OF HYDRAULICS SECTION, St. Paul District

Responsible for hydraulic engineering associated with the planning, design,construction, operation, and maintenance for various civil works water resources programs and projects.

The water resources areas covered include navigation, flood risk management,ecosystem restoration, dam and levee safety, bank stabilization, flood reconnaissance…….

The work includes: • modeling and design for levees, bank stabilization, channel improvements, interior flood control, habitat projects, and hydraulic structures• collecting and analyzing hydraulic and sediment transport data• ice reconnaissance and data collection• reconnaissance and support for flood emergencies • support to periodic inspections at locks and dams• wetlands permit reviews• dredge material management• coastal engineering• agency technical reviews

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Water Properties

• Heavy - 1 cubic foot weighs 62.4 lbs – Gasoline weighs 42.5 lbs/ft3– Rock weighs 110 lbs/ft3– Concrete weighs 165 lbs/ft3– Wood: White Oak = 48 lbs/ft3 – White Pine = 26 lbs/ft3– Temperature Affects: Important in lakes and reservoirs

• Why does something float?• The laws of buoyancy (Archimedes’ Principle, 250 BC)

and floatation are usually stated:– A body immersed in a fluid is buoyed up by a force equal to the

weight of fluid displaced– A floating body displaces its own weight of the liquid in which it

floats

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Water Properties• Flows from high energy to lower energy

zones• Follows the path of least resistance

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Hig

hw

ay 3

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New Channel Eroded During 2002 Flood

Dam

Water Properties• Water Seeks its Own level

– If it’s at elevation 100 on one side of a dam or levee, it wants to be the same elevation on the other side.

• Incompressible- So It Transmits Energy Well• Liquid at normal temperature• Heating to 212 °F (boiling point) raises the vapor

pressure to atmospheric pressure and liquid turns to gas• Lowering the pressure (for instance in a pipe or on a

spinning propeller) can cause gas formation at lower temperatures– Cavitation: Low pressure conditions results in liquid to gas and

then collapse back to liquid which plucks metal off turbine blades and concrete off spillways

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Water Properties• Density doesn’t change significantly with

ambient temperature range.– 1.9385 slugs/ft3 at 32 °F – 1.9388 slugs/ft3 at 40 °F (water is heaviest)– 1.9348 slugs/ft3 at 70 °F (0.2% lighter than at 40 °F)

• This means we can usually ignore density changes in flowing rivers because of mixing

• But in lakes these slight density differences result in thermal stratification

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Water at Rest

• Pressure (P) Increases with Depth

P = 62.4 lbs/ft3 x depth ft

Total Force acts at 2/3 total depth

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Water in Motion

• Water moves from higher to lower energy– Gravity (or downhill) flow– Pumps can be used to increase the energy levels to

move water to a higher level or to get it over a barrier• The flow of water is resisted by the boundary

roughness. This is true in channels and in pipes

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Downstream Flow

ResistanceChannel Bottom

FlowDirection

Water in Motion

River Bank

River Bank

A

v

The flow rate (Q) of water moving past a cross section is equal to the area (A) of the cross section multiplied by the velocity (v)

Q (ft3/s) = A (ft2) * v (ft/s)

Flow rate is also called discharge andcfs, or cubic feet per second, is used to designate flow

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Ways to describe water in motion

• The next few images attempt to define common terms that H&H people use when describing the movement of water.

• In a multi-discipline team setting you don’t need to be an expert in H&H, but you should understand some of the lingo.

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PrimaryFlowDirection

Secondary Current

One-, Two-, and Three-Dimensional Flows

x

y

z

Truth is that when water is flowing it is almost always 3-dimensional.But that is difficult to quantify.So we like to simplify, if we can, to 2-dimensional or 1-dimensionalin our descriptions, analysis, and modeling.

River Bank

River Bank

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One-, Two-, and Three-Dimensional Flows

• When planners talk to hydraulic modelers they will hear the terms 1-D, 2-D, 3-D flow and models.– In a one-dimensional flow, the change of fluid variables (velocity,

temperature, etc.) in one direction dominates over the change in the other two directions.

– In two- and three-dimensional flows, the change in fluid variables is important in multiple directions.

– Coefficients, model calibration, and past experience are used to account for simplifying assumptions.

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Change of Flow with Respect to Time

- no change: steady- changing: unsteady

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Steady Flow Example

• Release at a spillway notch with a constant headwater.

• Q = constant

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Unsteady Flow Example• Missouri River Flow hydrograph at Decatur• Q varies with time

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Change of Flow Conditions with Respect to Spatial

Position?

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Varied Flow Conditions

Dam

M1 Backwater Profile

Abrupt drop, downstream channelization

M2 Backwater Profile

Flow along these profiles is gradually varied (ie. the flow characteristics change slowly in the upstream to downstream direction). Right at the abrupt drop in the lower profile however the flow characteristics change vary fast. This is known as rapidly varied flow

Flow Direction

Flow Direction

Channel Bottom

Channel Bottom

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Change in modeled velocity with space and discharge

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Change in Velocity Across River Valley During a Flood

Velocity & Water Depth in Upper Pool 10, RM 639.6, April 17, 1997 (25-year flood)

-40-35-30-25-20-15-10-505

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0

837

1606

2255

3213

3615

3774

4191

4795

5036

5291

5489

5802

6203

6467

6996

7499

7837

Cross Valley Station (Feet From Wisconsin Shoreline)

Wa

ter

De

pth

(ft

) &

Ve

loc

ity

(fp

s)

Depth Velocity21

Specific Energy (Open Channels)

• For open channels, the specific energy can be defined as

E = y + v2/2g, where y is the depth of water above the channel bed, v is the flow velocity, g is the acceleration of gravity.

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Downstream Flow

Channel Bottom

y

v2/2g

Normal Depth

• the depth for which available energy and energy expended are in balance

Downstream Flow

ResistanceChannel Bottom

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Seeking Normal Depth

Dam

M1 Backwater Profile

Abrupt drop ordownstream channelization

M2 Backwater Profile

Given a sufficient channel length of unchanging section size,shape, slope, and roughness, depth tends to approachand/or maintain normal depth at some point upstream.

Flow Direction

Flow Direction

Channel Bottom

Channel Bottom

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Critical Depth

• the depth at which energy is the minimum possible for an open channel cross section of a given size and shape and a given flow

• the VALUE of critical depth does not depend on roughness or slope, (only on discharge)

yc = (q2/g)1/3 (Rectangular Channels)

25E

y

yc

Emin

Open Channel Flow Regimes• Subcritical Flow:

– is deeper and slower than critical flow

– this is what you usually see in rivers.

– relatively slow moving

• Supercritical Flow:– is shallower and faster than

critical flow – this is what you see in

steep channels (rapids or flow over a dam)

– fast moving

Rio Puerco, NM

St. Anthony Falls, Miss R, MN

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Open Channel Flow Regimes• Hydraulic jumps occur when there is a transition

from supercritical to subcritical flow

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Continuity Equation

Q = V1A1=V2A2

( Flow = Velocity X Area )

QA1

V1

A2

V2

Q

1

2

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How do you calculate velocity

• What are some drivers (or parameters) that affect the speed that water moves at?

• What would a basic relationship look like?

v = ?

FlowDirection

River Bank

River Bank

A

v

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FlowDirection

PAVariable ListA = Cross Sectional Area (ft2)P = Wetted Perimeter (ft)R = A/P = Hydraulic Radius (ft)n = boundary roughness Q = Discharge (ft3/s)s = water surface slope = hl/Lv = velocity (ft/s)

Mannings Equation (English Units)v = 1.49R2/3 s1/2 / n Q = 1.49AR2/3 s1/2 / n

L

hLRiver B

ank

River Bank

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FlowDirection

PcAc

Same Variable List (subscripts c and f)A = Cross Sectional Area (ft2)P = Wetted Perimeter (ft)R = A/P = Hydraulic Radius (ft)n = boundary roughnessQ = Discharge (ft3/s)s = water surface slope

Mannings Equation with Floodplain FlowQtotal = Qchannel +QfloodplainQtotal = 1.49AcRc

2/3 s1/2 / nc + 1.49AfRf2/3 s1/2 / nf

AfPf

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Closed Conduits – The area of flow is knownwhen it is flowing full – though closed conduitflow is often in the open channel mode (e.g.Many culverts, storm sewers)

Open Channel: Free surface varies with time affecting area, channel area, depth,roughness, and slope. The relationship between all these variables, usually makesopen channel flow problems tougher to deal with.

Cross Sections

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Closed Conduits – Roughness varies depending on material types, age, andcondition of pipe.

Open Channel: roughness varies depending on water depth, substrate size,density of vegetation, …..

Roughness (n-values)

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Hydraulic Radius, R

R = A/P where A = area (ft2) and P = Perimeter (ft)

Square Pipe Flowing Full A = L2 P = 4L R = L2/4L = L/4 where L = side length (ft)

Circular Pipe Flowing Full A = πD2/4 P = πD R = πD2/4 = D/4 πD where D = diameter (ft)

Open channels where width is much greater than depth A = by P = b+2y R = by/(b+2y), but 2y<<b so R ≈ y

L

D

by 34

Slope, s

• this is the friction slope, i.e. the rate at which head (energy) is expended through friction per unit horizontal length of flow, ft/ft

• the physical slope of a pipe or the bed of an open channel may, or may not, happen to match the friction slope

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Open Channel Flowlines

- a plot of water surface elevationover distance

- calculated by numerical models or by using existing gage data

-multiple flows: a family of flowlines

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610

620

630

640

650

660

670

680

630 635 640 645 650 655 660 665 670 675 680 685 690 695 700 705 710 715 720 725 730 735 740 745 750 755 760 765 770

River Mile

WS

EL

(N

GV

D 1

912)

Post Lock 2 yr Flow Post Lock 25% Flow Post Lock 75% Flow

Lo

ck a

nd

Dam

4

Ro

ot

Riv

er

Up

per

Iow

a R

iver

Ch

ipp

ewa

Riv

er

Wis

con

sin

RIv

er

Lo

ck a

nd

Dam

5A

Lo

ck a

nd

Dam

5

Lo

ck a

nd

Dam

7

Lo

ck a

nd

Dam

6

Lo

ck a

nd

Dam

9

Lo

ck a

nd

Dam

8

Bla

ck R

iver

Zu

mb

ro R

iver

Family of Flow Lines

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Changes That Tendto Raise a Flowline

- greater flow- greater roughness

- lesser slope- higher downstream water surface

- constricted area

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Tributary-Main Stem Interaction

Mainstem high flow = M1 on Trib.

Mainstem low flow = M2 on Trib.

Channel Bottom

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Bridge 6477TH 75 over Wild Rice River

Flood 4/18/1997 Backwater of Red River

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Bridge 6477Flood 6/2002 over 500-year event

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Change in floodplain features and change in Manning’s n with time due to dam

construction, wind, river currents, ice

Open Water in 1992n = .03Marsh in 1956

n = .05

Trees, Shrubs, Grass in 1900n = .1

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Slope of the lower 90 miles of the Wild Rice River, Western MinnesotaThalweg Data for WRR

820

870

920

970

1020

1070

1120

1170

0 10 20 30 40 50 60 70 80 90 100River Mile

Ele

vati

on

in F

eet

abo

ve M

.S.L

. (N

VG

D 1

988)

2003/2006

Project Reach Ecosystem Restoration

River Miles Elevation Decrease per mile 0 to 30 1.4'30 to 40 2.5'40 to 50 4.6'50 to 60 7.0'60 to 80 5.9'80 to 90 3.4'

N

MILES

Ada Mahnomen Glacial

Moraine Region

Beach Ridge Region

Glacial Lake Plain Region

0 10 20

Twin Valley

Figure 3: Wild Rice River basin topographic regions

WRRFS primary focus

Twin

Ada

Plain Glacial

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Stage – Discharge Relationship

LOCK & DAM NO. 8 OPERATING CURVES

629.0

630.0

631.0

632.0

633.0

634.0

635.0

636.0

637.0

638.0

639.0

640.0

641.0

642.0

643.0

644.0

645.0

646.0

647.0

648.0

649.0

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Discharge in 1,000 cfs

Ele

vati

on

in

fee

t

Tailwater No. 7

LaCrosse, WI(Control Point)

Brownsville, MN

Headwater No. 8

Tailwater No. 8

Upper Mississippi RiverNine-Foot Navigation Channel

Lock & Dam No. 8Operating Curves

(Historic Record 1972-2001)

U.S. Army Corps of EngineersSt. Paul District - St Paul, MN

Plate 7-1

SECONDARYCONTROL

DAM IS OUT OF CONTROLEL. 630.0

PRIMARYCONTROL

Project PoolElevation 631.0

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Closed Conduits or Pipes

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• Flow through a pipe can be calculated using Bernoulli’s equation which relates available energy and pipe size to the resistance caused by friction within the pipe. In the diagram the energy is higher at section 1 than at section 2, resulting in flow from 1 to 2, but because of the roughness of the pipe walls, there is resistance.

p1/γ + v12/2g + z1 = p2/γ + v2

2/2g + z2 + hl1-2

Pipe or Culvert

Closed Conduits Flowing Full

1 2

Resistance 46

H

Frictional Losses

Reservoir 1

Reservoir 2

Pipe or Culvert

Closed Conduits Flowing Full

• Resistance can be divided into frictional loss, and minor losses caused by the entrance, gates, transitions, etc

• Mannings equation can be used (as opposed to the Bernoulli form of the equation) to solve problems

Entrance LossesExit Losses

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1. Culvert area (A) is known2. If flow is known, then velocity (v) is known, since Q = vA3. With velocity known, the Manning equation can be used to

calculate the hydraulic slope (s) to overcome friction in the pipe. v, R, and n are known, so solve for s.

v = 1.49R2/3s1/2/n

4. Since s is equal to H/L, and L is known, then solve for H, or the difference in water surface from one side of the road to the other.

H

UpstreamSide

DownstreamSide

Culvert

Closed Conduits Flowing Full

L

Top of Road

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Culvert Reality

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Culvert Reality

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Additional Hydraulic Topics

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Open Channel --The Weir

- doubling head more thandoubles the flow. Look at the

equation

Q = CBH3/2

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• Doubling head more than doubles the flow

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Q = CBH3/2

C = ConstantB = Width H = Head

Closed Conduit Example--The Orifice

- head must be more than doubledto double the flow. Look at the

equation

Orifice: Q = CA(2gH)1/2

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Bankfull Discharge

FlowDirection

PcAc

AfPf

Stage

DischargeQb

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Temperature Effects on WaterWind Mixed Layer

Temp

Temp

Lakes, Summer

Rivers

Downstream Surface Wedge

Density Underflow Dam

Outflow

Reservoirs

Temp

Temp

Wind Mixed Layer

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Thermocline

Ice Effects

• Winter temperature profile – warmer water near the bottom. Ice is less dense than water (ice cubes float after all)

• Ice Damage – describe ice expansion-it happens in your freezer.

• Ice Jams• Frazil Ice – super-cooled water• Frozen subsurface water in soil reduces

infiltration the next Spring• Ice formation on steel structures, barges

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Lake Onalaska Outside Dredge Cut 2/8/08

WinterTemperatureCriterion > 1 C

WinterVelocityCriterion < 0.03 ft/s

Winter DOCriterion > 3 mg/L

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Ice Damaged Groin, Trempealeau National Wildlife Refuge

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As-built Profile

Root River Ice Jam

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Lock and Dam 2, Debris, 2001 Flood

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Wind-Driven Wave Damage During a Flood

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Groundwater/Surface Water Interaction

• Rivers re-charge groundwater table when river is high

• Groundwater recharges rivers when river is low • Balance can be upset by dams, sediment

deposition, groundwater pumping,…• At reach scale (ie. study area is a small reach of

a much larger river or stream), groundwater’s contribution to open channel flow is ignored.

• At a watershed scale, groundwater’s influence to open channel flow can be significant

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Measuring Flow

• Velocity – Price, ADCP• Discharge• Stage• Wind – waves• Flow characteristics through a structure

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Measurement Options

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Measurement Options

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Hydraulic Surveys• Boat with dual 50 HP

motors• Fathometer• GPS• Laptop• ADCP (Acoustic

Doppler)• Boat Safety

Equipment and Training

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~ The End ~

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